As an alternative to surgical treatment of pathological tissue wherein the diseased region is physically removed from the body, there is an increased interest in treating tissue in situ with a minimally invasive process. One such process that can selectively treat diseased tissue in a non-invasive manner is high intensity focused ultrasound (HIFU). With HIFU, high intensity acoustic signals are directed at a target treatment site in order to subject the tissue to a rapid increase in temperature and/or to mechanical destruction due to interaction with the applied acoustic signals. The treated tissue may form one or more “lesions” that are typically left in the body and may be absorbed through normal physiological processes.
When HIFU therapy is applied to a desired tissue site, variations in tissue depth and other properties such as diffraction, attenuation, sound speed, or other tissue-related parameters along the acoustic propagation path affect the amount of energy deposited. These variations cause corresponding variations in the size and nature of the resulting lesions created. Treatment regimens that are solely based on applying a predetermined dose of HIFU energy may therefore achieve inconsistent results due to these variations.
As an example, the transabdominal treatment of uterine fibroids with HIFU requires passing acoustic HIFU energy through multiple tissue layers of varying depth that have diverse properties (e.g. skin, fat, muscle, fluid in bladder, uterine wall and the fibroid itself). If not carefully controlled, the application of HIFU energy to the treatment site may cause undesired damage to tissue surrounding the fibroid as well as inconsistent results within the treatment site.
Prior art in the field of “cavitation detection” has included monitoring the amplitude and/or energy of bubble reflections in an attempt to monitor the progression of HIFU treatment. One typical implementation of cavitation detection in prior art is to halt or reduce application of HIFU power upon the first instance of bubble detection at any point along the treatment path, in order to avoid distorted or exaggerated lesion volumes that often result from the onset of uncontrolled cavitation or boiling. However, this usage can severely limit the efficacy of resultant HIFU lesions because this type of indiscriminant bubble detection without regard for the spatial distribution of such bubbles can result in premature cessation of HIFU power long before the lesion has filled the desired target volume.
Another proposed technique used in prior art involves the use of standard imaging ultrasound to monitor the “hyperecho” (i.e. reflections off bubbles in the tissue that are displayed as regions of enhanced brightness on standard B-mode ultrasound images). Visual observation of the hyperecho, however, does not quantify the amplitude or energy of the signal reflected from a given region and is therefore also unreliable in predicting the location and extent of the HIFU lesions created.
Finally, some HIFU therapies rely on MRI to monitor temperature as a proxy for HIFU lesion formation. However, this method is extremely expensive (MRI systems typically cost $1-3M), it is not real time since several seconds are required between MRI acquisitions, and the measured temperature is not considered accurate enough to automatically control HIFU parameters such as treatment duration.
Given these problems, there is a need for a more reliable and cost effective method to monitor the formation of HIFU lesions at varying depths and in tissue with statically or dynamically varying properties.